5 month old teething and loss of appetite

5 month old DS is formula fed, 3 weeks ago put him up to 7oz x 5 per day, seemed to go down with gusto. Anyway, to the point, he got a cold last week and it seemed to knock him a bit wrong, he was drinking 5 -7oz per feed, with a bit of a struggle, but to be expected. Since then he hasn't really pulled it back and each feed is becoming more of a struggle, just about scraping 5oz in, and I think this is coupled with teething (flushed cheeks, lots of finger sucking and hand chewing). Yesterday there was a couple of incidences where as soon as the bottle reached his mouth he was writhing and crying. I gave him some calpol which helped and he drank about 6oz (delighted!). He was upset again during his dream feed, again some calpol seemed to sort. This morning he's managing just about 4oz each time and that's a battle - but no crying or calpol , thankfully. It's also worth mentioning I've tried the teething gel with no luck before feedings (well, the ones where he was getting distressed)

Thing is , when do I worry?? He seems to have cut his feeding by almost half and I'm not sure what to do! He seems happy enough and poo-ing and wee-ing ok. But is it enough? How long do I let this go on for?